1. Field of the Invention
The present invention relates to a pneumatic tire for traveling on an off road which is used on an off road such as a desert, a gravel road, a dirt road, and a rocky place, like a tire for a desert race, a mud and snow tire.
2. Description of the Related Art
Since the tire for traveling on the off road is mainly used on the off road such as the desert, the gravel road, the dirt road, the rocky place, it is required to prevent a puncture caused by a stone-biting. Accordingly, there has been employed a shape in which a cross sectional shape of a groove is formed in a V-shaped form, so that a pebble and the like are hard to be pinched and tend to be detached. However, in the V-shaped groove, a puncture resistance is not sufficient, and a traction performance at an end stage of abrasion also becomes insufficient.
On the other hand, for a tire for a truck which frequently moves on the gravel road or the like, as a technique for preventing the puncture caused by the stone-biting, there has been known a method of setting a projection at a groove bottom. For example, in Japanese Unexamined Patent Publication No. 2000-43513, there is disclosed a structure in which a projection continuing in a tire circumferential direction at a groove bottom of a shoulder portion adjacent to a block. Further, in Japanese Unexamined Patent Publication No. H10-35224, there is disclosed a structure in which a projection having a larger height and a projection having a smaller height are alternately provided at a groove bottom of a groove formed in the periphery of the block.
However, in both of the tires mentioned above, since opposing groove wall surfaces of the groove provided with the projection rise up linearly, the pebble and the like tend to be pinched between the opposing groove wall surfaces and are hard to be detached, resulting in that a depression effect of the puncture caused by the stone-biting is small. Further, in the structure in which the large and small projections are alternately provided at the groove bottom such as disclosed in Japanese Unexamined Patent Publication No. H10-35224, the puncture resistance and the traction performance at the end stage of abrasion are not to be sufficient.